The containment for a nuclear reactor is defined as the enclosure that provides environmental isolation to the nuclear steam supply system (NSSS) of the plant in which nuclear fission is harnessed to produce pressurized steam. A commercial nuclear reactor is required to be enclosed in a pressure retaining structure which can withstand the temperature and pressure resulting from the most severe accident that can be postulated for the facility. The most severe energy release accidents that can be postulated for a reactor and its containment can be of two types.
First, an event that follows a loss-of-coolant accident (LOCA) and involve a rapid large release of thermal energy from the plant's nuclear steam supply system (NSSS) due to a sudden release of reactor's coolant in the containment space. The reactor coolant, suddenly depressurized, would violently flash resulting in a rapid rise of pressure and temperature in the containment space. The in-containment space is rendered into a mixture of air and steam. LOCA can be credibly postulated by assuming a sudden failure in a pipe carrying the reactor coolant.
Another second thermal event of potential risk to the integrity of the containment is the scenario wherein all heat rejection paths from the plant's nuclear steam supply system (NSSS) are lost, forcing the reactor into a “scram.” A station black-out is such an event. The decay heat generated in the reactor must be removed to protect it from an uncontrolled pressure rise.
More recently, the containment structure has also been called upon by the regulators to withstand the impact from a crashing aircraft. Containment structures have typically been built as massive reinforced concrete domes to withstand the internal pressure from LOCA. Although its thick concrete wall could be capable of withstanding an aircraft impact, it is also a good insulator of heat, requiring pumped heat rejection systems (employ heat exchangers and pumps) to reject its unwanted heat to the external environment (to minimize the pressure rise or to remove decay heat). Such heat rejection systems, however, rely on a robust power source (off-site or local diesel generator, for example) to power the pumps. The station black out at Fukushima in the wake of the tsunami is a sobering reminder of the folly of relying on pumps.
Present day containment structures with their monolithic reinforced concrete construction make it extremely difficult and expensive to remove and install a large capital requirement such as a steam generator in the NSSS enclosed by them. To make a major equipment change out, a hatch opening in the thick concrete dome has to be made at great expense and down time for the reactor. Unfortunately, far too many steam generators have had to be changed out at numerous reactors in the past 25 years by cutting through the containment dome at billions of dollars in cost to the nuclear power industry.
In a nuclear plant, the component cooling water (CCW) system is a closed loop of purified water that serves to cool a variety of equipment in the plant. Among its important auxiliary roles is extracting the decay heat from the reactor water after the reactor is shutdown, which is typically performed inside a tubular heat exchanger known variously as the “decay heat cooler” or “residual heat removal heat exchanger.” The heat transferred to component cooling water in the decay heat cooler and other heat exchangers that are used to cool electrical and mechanical machinery occurs across the walls of tubes which sequesters or isolates the component cooling water from the radioactive contamination that may be associated with the reactor water. Thus, the component cooling system essentially serves to provide the means to remove waste heat from all equipment in the plant that requires cooling as well as to serve as a barrier against release of radiation to the environment.
The heat collected by the component cooling water from plant equipment, however, raises its temperature. The heated component cooling water is typically cooled in a once-through flow system by rejecting its heat to the environment in a shell-and-tube heat exchanger using a natural body of water such a lake, river, or sea. The component cooling water system draws cool raw water from the natural body of water, which is pumped through the component cooling water heat exchanger and then returns the now heated water back to the natural body of water. Such a CCW system, however, suffers from several operational problems such as intrusion of debris carried over by the raw cooling water, biological fouling of heat exchanger tubes by raw water, and corrosion of pipes carrying the raw water into the heat exchanger. Operating nuclear plants often report significant accumulation of sediments and other foulants in the headers of CCW heat exchangers requiring frequent maintenance and degrading thermal performance.
The above weaknesses in the state-of-the-art call for an improved nuclear reactor containment system and component cooling water system.